Fix memory leak

VTK objects were just being set to nullptr instead of properly
getting deleted.
latest
Status Job ID Name Coverage
  External
passed cdash-buildbot

passed cdash-commit

passed ghostflow-branch-check-master

failed vtk@master-adora-linux-shared-release+cuda+gcc+mpi+optix+python2+tbb+vtkm

01:21:37

failed vtk@master-bigmac-osx-shared-debug+adios2+clang+python2+vtkm

04:29:44

failed vtk@master-duma-linux-shared-release+osmesa+ospray

00:23:08

running vtk@master-eeloo-linux-shared-release+adios2+extdeps+java+mpi+python3+qt+tbb+vtkm

3905:21:10

running vtk@master-eeloo-linux-shared-release+mpi+nogl

3905:21:09

failed vtk@master-luigi-linux-shared-release+mpi+python2+tbb

00:51:47

failed vtk@master-mun-windows-shared-release+kits+mpi+python3+qt+tbb

00:58:18

failed vtk@master-mun-windows-shared-release+mpi+python3+tbb+vtkm

00:25:09

failed vtk@master-mun-windows-static-release+mpi+qt+tbb

00:56:12

failed vtk@master-taanab-linux-shared-release+adios2+mpi+python2

00:22:20

failed vtk@master-trey-osx-shared-release+python3+qt

10:53:19